Housing for simple assembly of an ewod device

ABSTRACT

An EWOD device includes a first substrate assembly and a second substrate assembly; wherein one of said substrate assemblies includes electrowetting electrodes, and the first substrate assembly and the second substrate assembly are spaced apart to define a channel between the substrate assemblies; and a housing for receiving the first substrate assembly and the second substrate assembly, the housing comprising an alignment feature for locating at least one of the first and second substrate assemblies within the housing. The device further includes a fixing feature for fixing the first and second substrate assemblies within the housing. The second substrate assembly is located within the housing such that the second substrate assembly is an outer component of the EWOD device. The device further may include a spacer that spaces apart the first substrate assembly from the second substrate assembly to define the channel between the first and second substrate assemblies.

TECHNICAL FIELD

The present invention relates to droplet microfluidic devices and theirconstruction, and more specifically to Active MatrixElectro-wetting-On-Dielectric (AM-EWOD) devices, including housingstructures for simplified assembly and configurations of such devices.

BACKGROUND ART

Electrowetting on dielectric (EWOD) is a well-known technique formanipulating droplets of fluid by the application of an electric field.Active Matrix EWOD (AM-EWOD) refers to implementation of EWOD in anactive matrix array incorporating transistors, for example by using thinfilm transistors (TFTs). It is thus a candidate technology for digitalmicrofluidics for lab-on-a-chip technology. An introduction to the basicprinciples of the technology can be found in “Digital microfluidics: isa true lab-on-a-chip possible?”, R. B. Fair, Microfluid Nanofluid (2007)3:245-281).

FIG. 1 shows a part of a conventional EWOD device in cross section. Thedevice includes a lower substrate 10, the uppermost layer of which isformed from a conductive material which is patterned so that a pluralityof array element electrodes 12 (e.g., 12A and 12B in FIG. 1) arerealized. The electrode of a given array element may be termed theelement electrode 12. A liquid droplet 14, including a polar material(which is commonly also aqueous and/or ionic), is constrained in a planebetween the lower substrate 10 and a top substrate 16. A suitable gap orchannel between the two substrates may be realized by means of a spacer18, and a nonpolar surround fluid 20 (e.g. oil) may be used to occupythe volume not occupied by the liquid droplet 14. The function of theoil is to reduce the surface tension at the surfaces of the polardroplets, and to increase the electro-wetting force, which ultimatelyleads to the ability to create small droplets and to move them quickly.It is usually beneficial, therefore, for the oil to be present withinthe channel of the device before any polar fluids are introducedtherein.

An insulator layer 22 disposed upon the lower substrate 10 separates theconductive element electrodes 12A, 12B from a first hydrophobic coating24 upon which the liquid droplet 14 sits with a contact angle 26represented by θ. The hydrophobic coating is formed from a hydrophobicmaterial (commonly, but not necessarily, a fluoropolymer). On the topsubstrate 16 is a second hydrophobic coating 28 with which the liquiddroplet 14 may come into contact. Interposed between the top substrate16 and the second hydrophobic coating 28 is a reference electrode 30.

The contact angle θ is defined as shown in FIG. 1, and is determined bythe balancing of the surface tension components between the solid-toliquid (γ_(SL)), the liquid-to non-polar surrounding fluid (γ_(LG)) andthe solid to non-polar surrounding fluid (γ_(SG)) interfaces, and in thecase where no voltages are applied satisfies Young's law, the equationbeing given by:

$\begin{matrix}{{\cos \; \theta} = \frac{\gamma_{SG} - \gamma_{SL}}{\gamma_{LG}}} & \left( {{equation}\mspace{14mu} 1} \right)\end{matrix}$

In operation, voltages termed the EW drive voltages, (e.g. V_(T), V₀ andV₀₀ in FIG. 1) may be externally applied to different electrodes (e.g.reference electrode 30, element electrodes 12, 12A and 12B,respectively). The resulting electrical forces that are set upeffectively control the hydrophobicity of the hydrophobic coating 24. Byarranging for different EW drive voltages (e.g. V₀ and V₀₀) to beapplied to different element electrodes (e.g. 12A and 12B), the liquiddroplet 14 may be moved in the lateral plane between the two substrates10 and 16.

Example configurations and operation of EWOD devices are described inthe following. U.S. Pat. No. 6,911,132 (Pamula et al., issued Jun. 28,2005) discloses a two dimensional EWOD array to control the position andmovement of droplets in two dimensions. U.S. Pat. No. 6,565,727(Shenderov, issued May 20, 2003) further discloses methods for otherdroplet operations including the splitting and merging of droplets, andthe mixing together of droplets of different materials. U.S. Pat. No.7,163,612 (Sterling et al., issued Jan. 16, 2007) describes how TFTbased thin film electronics may be used to control the addressing ofvoltage pulses to an EWOD array by using circuit arrangements verysimilar to those employed in AM display technologies.

The approach of U.S. Pat. No. 7,163,612 may be termed “Active MatrixElectrowetting on Dielectric” (AM-EWOD). There are several advantages inusing TFT based thin film electronics to control an EWOD array, namely:

-   -   Electronic driver circuits can be integrated onto the lower        substrate 10.    -   TFT-based thin film electronics are well suited to the AM-EWOD        application. They are cheap to produce so that relatively large        substrate areas can be produced at relatively low cost.    -   TFTs fabricated in standard processes can be designed to operate        at much higher voltages than transistors fabricated in standard        CMOS processes. This is significant since many EWOD technologies        require electro-wetting voltages in excess of 20V to be applied.

FIG. 2 is a drawing depicting additional details of an exemplary AM-EWODdevice 36 in schematic perspective, which may incorporate the layeredstructures in FIG. 1. The AM-EWOD device 36 has a lower substrate 44with thin film electronics 46 disposed upon the lower substrate 44, anda reference electrode (comparable to reference electrode 30 above) isincorporated into an upper substrate 54. The electrode configuration maybe reversed, with the thin film electronics being incorporated into theupper substrate and the reference electrode being incorporated into thelower substrate. The thin film electronics 46 are arranged to drivearray element electrodes 48. A plurality of array element electrodes 48are arranged in an electrode or element array 50, having X by Y arrayelements where X and Y may be any integer. A liquid droplet 52 which mayinclude any polar liquid and which typically may be aqueous, is enclosedbetween the lower substrate 44 and the upper substrate 54 separated by aspacer 56, although it will be appreciated that multiple liquid droplets52 can be present.

There are various configurations of housings utilized in theconstruction of EWOD devices, to support the substrates, internalelectronics, and related components. For example, U.S. Pat. No.9,011,662 (Wang et al, issued Apr. 21, 2015) describes an EWODcartridge, including molded plastic components and active EWODcomponents, and associated assembly techniques. In such assembly, thelower EWOD substrate is always enclosed in some fashion. Typically, suchassembly employs a two-part molded structure in which the separate partsare formed and subsequently fused together via ultrasonic welding, or asingle part plastic piece is formed and then crimped around the lowerEWOD substrate to hold the lower substrate in position. Also, a fluidseal is formed via the use of a gasket formed as part of the upperplastic part via over-molding. The configuration and related assembly ofstructures such as those described in U.S. Pat. No. 9,011,662, which istypical of the conventional art, results in a substantial number ofcomplicated and expensive production techniques.

In conventional assembly techniques, typically the top and bottomsubstrates may be attached together on a large mother-glass from whichmultiple devices may be formed. Such an assembly process typicallyincludes the attachment of top and bottom mother-glass substrates, andthe formation of the spacer and seal regions on each individual device.Following the spacer and seal formation, the mother-glass may be cutinto individual assembled devices, e.g. using a scribe and break methodfor cutting the glass substrates. Assembly at the mother-glass level hascertain advantages, in particular that it facilitates the assembly ofmultiple devices at a time.

However, there are also significant disadvantages of mother-glassassembly. Specialized handling machinery on the production line isrequired to handle the processing of mother-glass pieces of thenecessary large size. This specialized equipment is expensive and maynot be suitable for certain facilities depending on size. In addition,it is typically required to pattern the hydrophobic coating layers inthe region of the seal, for example to achieve good adhesion of sealantmaterial (e.g. glue) to each substrate. The manufacturing process forcreating a patterned hydrophobic layer at mother-glass scale is complex.Conventional housings and related manufacturing processes for EWODdevices, therefore, remain deficient.

SUMMARY OF INVENTION

The manner of assembly and construction of an EWOD or AM-EWOD deviceneeds to be simplified while maintaining high overall performance of theresultant device. Accordingly, the present disclosure describes anenhanced device construction of an EWOD or AM-EWOD device and relatedassembly methods that facilitate device construction and assembly toprovide a low-cost and simplified manner to assemble individual EWODdevices. Multiple devices, therefore, can be assembled efficientlywithout the disadvantages associated with conventional mother-glasslevel assembly.

As described above, an EWOD device (including an AM-EWOD device)typically includes two glass substrates which are held apart at a fixeddistance by a spacer which separates the substrates at a well-definedgap (referred to as the cell gap) to form the EWOD channel. The cell gapof the EWOD channel needs to be formed with a high degree of parallelismof the two substrates to define a highly uniform EWOD channel, as suchparallelism is a significant metric in the quality and functionality ofthe EWOD device. The more uniform the cell gap of the EWOD channel, thegreater the accuracy and reproducibility of the volume of the dropletsproduced by the device and the greater the utility as an automatic andaccurate droplet dispenser.

According to a common device construction, the spacer that is used tospace the EWOD substrates at a fixed distance from each other also actsas the means of holding or fixing those two substrates in positionrelative to each other. In conventional configurations, the spacer couldincorporate a double-sided adhesive, or the spacer could include a gluetrack with a certain percentage of spacer balls whose diameter definethe cell gap. Alternatively, the spacer could include a dry sheetmaterial cut to shape, with a thin layer of adhesive either side of thedry spacer. In these conventional configurations, the spacer performstwo distinct functions: (1) a spacing function—the spacer spaces apartthe EWOD substrates to form the EWOD channel with the desired cell gap;and (2) a fixing function—the spacer includes some form of structure(e.g., adhesive layer, glue) to fix the substrates together in the senseof holding the substrates in place so that the substrates cannot easilybe taken apart or move during operation. Alternatively, the spacing andfixing functions can be achieved separately. For example, if spaceallows, there can be a dry spacer that accurately sets the EWOD channelcell gap, and a separate glue track which adheres the substratestogether.

For the typical glass substrates, an additional surrounding plastichousing will very often be used to house the EWOD module. An additionalplastic housing may: improve the ruggedness and ease of handling of theEWOD device to avoid significant contact with the glass; improve theappearance of the overall device; improve the ease of fluid interfacingto the device, for example by facilitating pipette guides and docks inthe plastic, to aid fluid loading and extraction; and provide electricaland mechanical contact between the EWOD device and the drive electronicsso that the device can be driven with electrical signals, and heldstationary for the subsequent assay to be performed. Conventionally, useof a plastic housing in this manner has required a two-stage assemblyprocess, including: (1) assembly and adhesion of the two glasssubstrates into a module; and (2) incorporation of the module into theplastic housing.

This present disclosure describes an EWOD device of simplified assembly,including an enhanced construction of a plastic housing that facilitatesthe spacing and fixing of the glass substrates, i.e., the housingconfiguration facilitates both the spacing and the fixing functionsreferenced above. A dry spacer performs the spacing function to definethe cell gap, and a plastic housing is used to facilitate fixing thesubstrates in place. The spacing and fixing of the upper and lowersubstrates, and thus the definition of the cell gap of the EWOD channeldefined by the substrates, is facilitated by the plastic housing.Accordingly, the plastic housing includes alignment and fixing featuresto facilitate the construction of the assembly. Examples of suchfeatures include: lateral alignment features for lateral alignment ofthe top and bottom substrates; vertical alignment features forpositioning the substrates to define the vertical cell gap between thetop and bottom substrates; adherence features for fixing the componentsof the EWOD device in place, which may include some or all of (a)adhesive surfaces, (b) structures to define glue channel inputs, and (c)mechanical adherence features; and sealing features for sealing theinterior EWOD channel of the EWOD device for maintaining the inputtedfluids within the EWOD channel. In exemplary embodiments, the topsubstrate and/or the spacer features may be pre-fabricated as part of,or pre-assembled into, the plastic housing prior to assembly with thebottom substrate.

An aspect of the invention, therefore, is an enhanced EWOD device havinga housing with alignment features optimized for simplified assembly. Inexemplary embodiments, the EWOD device includes a first substrateassembly and a second substrate assembly; wherein one of the first orsecond substrate assemblies includes electrowetting electrodes, and thefirst substrate assembly and the second substrate assembly are spacedapart to define a channel between the first and second substrateassemblies; and a housing for receiving the first substrate assembly andthe second substrate assembly, the housing comprising an alignmentfeature for locating at least one of the first and second substrateassemblies within the housing. The device further includes a fixingfeature for fixing the first and second substrate assemblies within thehousing. The second substrate assembly is located within the housingsuch that the second substrate assembly is an outer component of theEWOD device. The device further may include a spacer that spaces apartthe first substrate assembly from the second substrate assembly todefine the channel between the first and second substrate assemblies.The spacer may be located between the first and substrate assemblies outof direct contact with the housing, or the spacer may be fixed to thehousing with the second substrate assembly being fixed to the spacer bythe fixing feature.

The alignment feature may comprise a plurality of alignment surfacesdefined by the housing to guide the locating of one or more of the firstsubstrate assembly, second substrate assembly, and spacer duringassembly. The fixing feature may include a fixing material, such as acurable adhesive or curable glue that may be applied to form one or morefixing layers as guided by the alignment feature, to fix the componentswithin the housing.

Another aspect of the invention is a related method of assembling suchan EWOD device. In exemplary embodiments, the method may include thesteps of: providing a housing for receiving a first substrate assemblyand a second substrate assembly, the housing comprising an alignmentfeature for locating at least one of the first and second substrateassemblies within the housing; locating the first substrate assemblywithin the housing; locating the second substrate assembly within thehousing, wherein at least one of the first and second substrateassemblies is located within the housing using the alignment feature;and applying a fixing material to form at least one fixing layer to fixthe first and second substrate assemblies within the housing. The methodof assembly further may include locating a spacer within the housing,wherein the spacer spaces apart the first substrate assembly from thesecond substrate assembly to define the channel between the first andsecond substrate assemblies.

In exemplary embodiments, the first and second substrate assemblies, andthe spacer when present, all are located within the housing prior toapplying the fixing material that fixes the second substrate assembly inplace. After locating the first and second substrate assemblies, and aspacer when present, within the housing, portions of the housing arespaced apart from an edge of the second substrate assembly to formfixing ports for applying the fixing material. The fixing material isapplied through the fixing ports.

These and further features of the present invention will be apparentwith reference to the following description and attached drawings. Inthe description and drawings, particular embodiments of the inventionhave been disclosed in detail as being indicative of some of the ways inwhich the principles of the invention may be employed, but it isunderstood that the invention is not limited correspondingly in scope.Rather, the invention includes all changes, modifications andequivalents coming within the spirit and terms of the claims appendedhereto. Features that are described and/or illustrated with respect toone embodiment may be used in the same way or in a similar way in one ormore other embodiments and/or in combination with or instead of thefeatures of the other embodiments.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a drawing depicting a conventional EWOD device incross-section.

FIG. 2 is a drawing depicting an exemplary AM-EWOD device in schematicperspective.

FIG. 3 is a drawing depicting a side cross-section view of an exemplaryEWOD device in accordance with embodiments of the present invention.

FIG. 4 is a drawing depicting an isometric view of an exemplary housingfor use in the EWOD device of FIG. 3.

FIG. 5 is a drawing depicting an exemplary method of assembling the EWODdevice of FIG. 3, with each sub-view FIGS. 5(a), 5(b), 5(c), 5(d), 5(e),and 5(f) depicting an exemplary step of such method.

FIG. 6 is a drawing depicting a side cross-section view of anotherexemplary EWOD device in accordance with embodiments of the presentinvention.

FIG. 7 is a drawing depicting a side cross-section view of anotherexemplary EWOD device in accordance with embodiments of the presentinvention.

FIG. 8 is a drawing depicting a side cross-section view of anotherexemplary EWOD device in accordance with embodiments of the presentinvention.

FIG. 9 is a drawing depicting a side cross-section view of anotherexemplary EWOD device in accordance with embodiments of the presentinvention.

FIG. 10 is a drawing depicting a plan view of an exemplary EWOD deviceillustrating a configuration of fixing ports in accordance withembodiments of the present invention.

FIG. 11 is a drawing depicting a plan view of an exemplary EWOD deviceillustrating another configuration of fixing ports in accordance withembodiments of the present invention.

FIG. 12 is a drawing depicting a side cross-section view of anotherexemplary EWOD device in accordance with embodiments of the presentinvention.

FIG. 13 is a drawing depicting a side cross-section view of anotherexemplary EWOD device in accordance with embodiments of the presentinvention.

FIG. 14 is a drawing depicting a side cross-section view of anotherexemplary EWOD device in accordance with embodiments of the presentinvention.

FIG. 15 is a drawing depicting a side cross-section view of anotherexemplary EWOD device in accordance with embodiments of the presentinvention.

FIG. 16 is a drawing depicting a side cross-section view of anotherexemplary EWOD device in accordance with embodiments of the presentinvention.

FIG. 17 is a drawing depicting a side cross-section view of anotherexemplary EWOD device in accordance with embodiments of the presentinvention.

FIG. 18 is a drawing depicting a plan view of an exemplary EWOD devicein accordance with embodiments of the present invention, including theaddition of fluid input structures.

DESCRIPTION OF EMBODIMENTS

Embodiments of the present invention will now be described withreference to the drawings, wherein like reference numerals are used torefer to like elements throughout. It will be understood that thefigures are not necessarily to scale.

FIG. 3 is a drawing depicting a side cross-section view of an exemplaryEWOD device 80 in accordance with embodiments of the present invention.The EWOD device 80 includes a first or upper substrate assembly 82 and asecond or lower substrate assembly 84 separated by a spacer 86. Forsimplicity of illustration of pertinent features, the individual layersof the EWOD device components are omitted. Accordingly, the first andsecond substrate assemblies may include the associated glass substratelayers, insulating layers, electrode layers, and related structures thatform the EWOD device as are known in the art. Typically, the second(lower) substrate assembly 84 constitutes the TFT substrate that wouldinclude the thin film electronics, with the first (upper) substrateassembly 82 incorporating the reference electrode. This, however, may bereversed with the first (upper) substrate assembly 82 constituting theTFT substrate and the second (lower) substrate assembly 84 incorporatingthe reference electrode.

The spacer 86 is configured to have a desired width or thickness so asto correctly position the substrates relative to each other to define anEWOD channel 88. The spacer 86 may rest on one of the substrates, suchas the lower substrate 84, and the spacer has at least regions 90 thatextend into the EWOD channel 88 and thus are in contact with bothsubstrates so as to properly space the substrates apart (i.e., performthe spacing function) to form the EWOD channel of a desired cell gap.Example materials for the spacer can include suitable rigid sheetplastics, such as for example polycarbonate, PET, polystyrene,polyester, polyimides (e.g. Kapton, Cirlex), or Mylar.

Generally, an aspect of the invention is an enhanced EWOD device havinga housing with alignment features optimized for simplified assembly. Inexemplary embodiments, the EWOD device includes a first substrateassembly and a second substrate assembly; wherein one of the first orsecond substrate assemblies includes electrowetting electrodes, and thefirst substrate assembly and the second substrate assembly are spacedapart to define a channel between the first and second substrateassemblies; and a housing for receiving the first substrate assembly andthe second substrate assembly, the housing comprising an alignmentfeature for locating at least one of the first and second substrateassemblies within the housing. The device further includes a fixingfeature for fixing the first and second substrate assemblies within thehousing. The second substrate assembly is located within the housingsuch that the second substrate assembly is an outer component of theEWOD device. The device further may include a spacer that spaces apartthe first substrate assembly from the second substrate assembly todefine the channel between the first and second substrate assemblies.The spacer may be located between the first and substrate assemblies outof direct contact with the housing, or the spacer may be fixed to thehousing with the second substrate assembly being fixed to the spacer bythe fixing feature.

The alignment feature may comprise a plurality of alignment surfacesdefined by the housing to guide the locating of one or more of the firstsubstrate assembly, second substrate assembly, and spacer duringassembly. The fixing feature may include a fixing material, such as acurable adhesive or curable glue that may be applied to form one or morefixing layers as guided by the alignment feature, to fix the componentswithin the housing.

Referring to FIG. 3, the EWOD device 80 further includes a plastichousing 92 with an exemplary configuration of the fixing feature asfixing layers of a fixing material. In the example of FIG. 3, the uppersubstrate assembly 82 is fixed to the plastic housing 92 via a firstfixing layer 94. In addition, the lower substrate 84 also is fixed tothe plastic housing 92 via a second fixing layer 96. By fixing bothsubstrate assemblies directly to the housing, the spacer 86 isessentially locked between the two substrate assemblies withoutrequiring any additional fixing mechanism for the spacer. As such, thespacer 86 of this embodiment also is referred to as a “dry spacer”meaning that there is no adhesive or other fixing mechanism applied toor otherwise associated specifically with the spacer 86. Rather, thepositioning and fixing of the spacer 86 is achieved as a result of thepositioning and fixing of the substrate assemblies relative to thehousing to define the cell gap of the EWOD channel 88. In thepositioning of this particular embodiment, the spacer is located betweenthe first and second substrates out of direct contact with the housingalong at least a portion of the spacer, as shown in FIG. 3.

The first and second fixing layers 94 and 96 may be formed in a varietyof ways, examples of which are set forth herein. It will be appreciatedthat such examples are non-limiting, and any suitable materials andconfigurations may be employed. For example, the fixing layers may beformed as a doubled-sided tape. Suitable tape configurations may includea tape material that has two layers of a modified solvent-acrylateadhesive on either side of a PET or PVC carrier film. Adhesive gluelayers may be employed as another example. Ultraviolet (UV) curableglues or UV curable adhesives in particular may be employed, such as forexample UV curable epoxy, acrylic, silicone, cyanoacrylate, andanaerobic glues or adhesives. Other curable glues or adhesives may beemployed, such as for example materials that are thermally cured, orcured by exposure to moisture, pressure, or via a chemical reaction oftwo-part mixtures.

FIG. 4 is a drawing depicting an isometric view of the exemplaryconfiguration of housing 92, as viewed generally from the underside.Generally, the housing 92 may be configurated for aligning the twosubstrate assemblies 82 and 84, and by result the dry spacer 86, withrespect to each other in the manner of an assembly jig. With suchconfiguration, the EWOD device 80 can be easily assembled at the singledevice level, without having to employ mother-glass level construction.

In exemplary embodiments, the housing 92 may include a plurality ofalignment features to properly align the first and second substrateassemblies and/or spacer. Referring to the example of FIGS. 3 and 4, thehousing 92 may include a first substrate alignment surface 100 and asecond substrate alignment surface 102. The housing 92 further mayinclude a first or upper outer surface 104, and a second or bottom outersurface 106. Generally, the referenced surfaces of the housing 92 form astepped alignment configuration on the underside of the housing 92.

With the stepped configuration of the housing 92, a significant featureis that the separation of the two substrate assemblies 82 and 84, withinthe expanse between the first substrate alignment surface 100 and thesecond substrate alignment surface 102, may be determined by thethickness of the dry spacer 86 to define the EWOD channel cell gap. Inother words, such separation generally is not constrained by any givenplastic housing configuration. Due to the manner of manufacturing oftypical plastic housings, non-uniformities can be present in thehousing. These non-uniformities can manifest as surface roughness, ordeviations from precise parallelism of the housing surfaces. Suchnon-uniformities of the housing can result in deviations of resultantparallelism of the substrate assemblies in the assembled EWOD device, inturn leading to non-uniformities of the cell gap of the EWOD channel.Accordingly, the design of a given housing 92 does not control the cellgap of the EWOD channel. Instead, the fixing plane of the housing wherethe second substrate alignment surface 102 is located (and where thesecond fixing layer 96 is applied) should be within a distance of thebottom substrate assembly that can be filled easily with the fixinglayer material, so that the fixing material (e.g., glue or adhesive)fills and accommodates any non-uniformities in the plastic of thehousing. As a result, a high level of parallelism of the substrateassemblies is achieved in the assembled EWOD device 80, resulting in ahighly uniform cell gap of the EWOD channel 88.

In addition, based on the stepped configuration of the alignmentfeatures, the second substrate assembly is located within the housingsuch that the second substrate assembly is an outer component of theEWOD device. In this particular example, outer surfaces of the secondsubstrate assembly and the housing are essentially flush with eachother, although this need not be the case.

To accurately apply the fixing layer 96 properly in the fixing planeformed by the second substrate alignment surface 102, the use of a lowviscosity curable adhesive or glue may be selected from among the fixingmaterials described above. With use of a low viscosity curable material,the material will flow freely to accommodate any non-uniformities of thehousing 92 at the fixing plane. The optional use of a suitable amount ofapplied pressure and/or applying pressure with a weighting block mayimprove the EWOD substrate parallelism. Referring to FIG. 3, with theuse of a low viscosity adhesive material, if the adhesive comes intocontact with the spacer, the adhesive material may wick underneath thespacer by capillary action, and therefore possibly enter into the EWODchannel of the device in a small amount. This does not interfere withthe operation of the EWOD device, and indeed may be useful for holdingthe spacer in position so that the spacer cannot move during deviceoperation.

FIG. 5 is a drawing depicting an exemplary method of assembling the EWODdevice 80, with each view (a)-(f) depicting an exemplary step of suchmethod. In a Step (a), housing 92 is provided and is oriented to accessthe underside having the alignment features described above. In a Step(b), the first fixing layer 94 may be applied to a fixing plane formedby the first substrate alignment surface 100. In a Step (c), the firstor upper substrate assembly 82 is positioned on the first fixing layer94 as supported by the first substrate alignment surface 100. In a Step(d), the spacer 86 is positioned against the upper substrate assembly 82in a “dry” manner, i.e., without the addition of any fixing materialapplied to the spacer. In a Step (e), the second fixing layer 96 may beapplied to a fixing plane formed by the second substrate alignmentsurface 102. In a Step (f), the second substrate assembly 84 ispositioned on the second fixing layer 96 as supported by the secondsubstrate alignment surface 102. Again, based on the steppedconfiguration of the alignment features, the second substrate assemblyis located within the housing such that the second substrate assembly isan outer component of the EWOD device in the resultant configuration. Inthis particular example, the second substrate assembly 84 resides at aposition essentially flush with the bottom outer surface 106 of thehousing (see also FIG. 3), and thus the second substrate assembly 84essentially is contained within the cornered alignment feature definedbetween the second substrate alignment surface 102 and the bottom outersurface 106. The assembled EWOD device 80 may then be cured to set thefixing layers in embodiments in which curable adhesives or curable gluesare employed as the fixing material.

The described configuration and manner of assembly of an EWOD or AM-EWOD80 is simplified as compared to conventional methods, while maintaininga highly uniform cell gap of the EWOD channel for effective performanceof the device. This is accomplished using common materials as are knownin the art, which are low cost and easy to manipulate for assembly ofindividual devices in an efficient manner. Multiple EWOD devices,therefore, can be assembled efficiently without the disadvantagesassociated with the costs and complexities of conventional mother-glasslevel assembly. The configuration of the housing and its relation to theother device components further performs both the spacing and the fixingfunctions in an effective manner to optimize the cell gap of the EWODchannel while fixing the various components in place.

Subsequent figures illustrate structural and assembly variations to theabove. For ease of illustration, like structures are identified withlike reference numerals as in the previous figures and generally arecomparably configured. Additional explanation is provided as to thestructural differences among the various embodiments. Relatedly, thevarious embodiments of the EWOD device generally will operate asdescribed above, with certain modifications based on the structuralvariations as detailed below.

FIG. 6 is a drawing depicting a side cross-section view of anotherexemplary EWOD device 80 a in accordance with embodiments of the presentinvention, having a variation of the fixing feature. In this embodiment,the first fixing layer 94 is omitted, such that the upper substrate 82is not adhered directly into position using any type of distinctadhesive or glue layer of fixing material. Rather, the upper substrate82 is simply placed into the associated alignment feature in the plastichousing 92 formed at the first substrate alignment surface 100. Thefixing layer 96 is present comparably as in the previous embodiment atthe second substrate alignment surface 102, to provide a fixing planefor adhering the second substrate assembly 84 to the housing 92.Accordingly, only adhesion associated with the bottom substrate 84 tothe housing 92 is maintained, as shown in FIG. 6. This embodiment may beappropriate when it is not required to form a seal between the uppersubstrate and the housing, for example when a user does not need toextract any fluid from the EWOD device during or at the end of the assayto be performed. For such circumstances, the embodiment of FIG. 6 has anadvantage of one less assembly step to be performed with less fixingmaterial being used, lowering the cost and complexity of assembly of theEWOD device.

FIG. 7 is a drawing depicting a side cross-section view of anotherexemplary EWOD device 80 b in accordance with embodiments of the presentinvention. In this embodiment, which may be preferable due to spacelimitations in certain situations, the lower substrate assembly 84 isnot fixed to the plastic housing directly, but is fixed to the housingindirectly via directly fixing the second substrate assembly to thespacer as shown in FIG. 7. In this configuration, the fixing featureincludes fixing layers that are provided on opposing faces of thespacer, and the fixing layer between the spacer and the lower substrateassembly may extend into the EWOD channel 88 as also shown in FIG. 7.

Referring to the example as depicted in FIG. 7, the EWOD device 80 b mayinclude a plastic housing 110. In the example of FIG. 7, the uppersubstrate assembly 82 is fixed to the plastic housing 110 via a firstfixing layer 112. A spacer 114 is shaped differently in this embodimentso as to extend to an edge of the bottom substrate 84 along at leastsome portions of the spacer. Accordingly, the spacer 114 in thisembodiment is fixed to the plastic housing 110 via a second fixing layer116, and lower substrate assembly 84 is fixed directly to the spacer 114via a third fixing layer 118. The fixing layers 116 and 118, therefore,are located on opposing surfaces of the spacer 114 to fix the spacerdirectly to both the housing 110 and the lower substrate assembly 84 atsaid opposing faces of the spacer. The fixing layers may be formedcomparably as in previous embodiments using comparable materials.

Because of the manner of fixing the spacer 114 via the second and thirdfixing layers 116 and 118, the stepped configuration of alignmentsurfaces may have somewhat different dimensions as compared to theembodiment of FIG. 3. Referring to the example of FIG. 7, the housing114 may include a first substrate alignment surface 120, which issimilar to the first substrate alignment surface 100 of previousembodiments. The housing 110 further may include a spacer alignmentsurface 122, which is dimensionally different from the second substratealignment surface 102 of previous embodiments insofar as the spacer isfixed to both the housing and the bottom substrate. During assembly,therefore, the second fixing layer 116 is applied to the spaceralignment surface 122, followed by placing of the spacer 114, followedby applying the third fixing layer 118. The bottom substrate layer 84 isthen fixed via the third fixing layer 118.

Alternatively, the second and third fixing layers may be incorporated aspart of the spacer 114 prior to assembly of the overall EWOD device 80b. This may be performed by forming the spacer as a double-sided tape,with adhesive tape layers being provided on opposing surfaces of a basespacer body. A double-sided tape spacer configuration may be employed inany of the embodiments described herein in which the spacer is fixeddirectly to both the housing 110 and the lower substrate assembly 84 atopposing faces of the spacer.

FIG. 8 is a drawing depicting a side cross-section view of anotherexemplary EWOD device 80 c in accordance with embodiments of the presentinvention, which bears structural similarity to the embodiment of FIG.3. In the embodiment of FIG. 8, portions 124 of housing 92 are spacedapart from an edge 126 of the bottom substrate 84 to form fixing ports128. During assembly, fixing material 125 for the second fixing layer 96may be applied through the fixing ports 128 to form said second fixinglayer 96 using an application instrument 127 (e.g., pipette orcomparable device). For the method of assembly in this embodiment, incontrast to previous embodiments, the fixing material, such as theadhesive or glue, is not applied to the bottom substrate or spacerbefore such components are inserted into the assembly. Rather, thespacer 86 and the bottom substrate assembly 84 are inserted dry into theposition for assembly, and then the fixing material is subsequentlyapplied to the device though the fixing ports 128. If a low viscosityglue or adhesive is used as the fixing material, it will fill into therelevant gaps by capillary action, and can be set properly upon curingafter the filling is complete. In this embodiment, it may beadvantageous to include the fixing ports within the plastic housing toallow the fixing material to be applied at the correct position tofacilitate the capillary filling, as shown in FIG. 8. The fixingmaterial can, for example, be applied automatically during the assemblyprocess, such as by using a programmable gluing robot.

Comparable principles may be applied to the assembly configuration ofFIG. 7, in which the housing and the bottom substrate are fixed toopposing faces of the spacer. Accordingly, FIG. 9 is a drawing depictinga side cross-section view of another exemplary EWOD device 80 d inaccordance with embodiments of the present invention, which bearsstructural similarity to the embodiment of FIG. 7. In the embodiment ofFIG. 9, the portions 124 of housing 92 likewise are spaced apart from anedge 126 of the bottom substrate 84 to form fixing ports. In addition,for this embodiment, it may be advantageous to have first fixing ports128 a and second fixing ports 128 b at different depths within thehousing, as shown in FIG. 9. In this manner, fixing ports 128 a may beemployed to form the second fixing layer 116, and fixing ports 128 b maybe employed to form the third fixing layer 118.

In the embodiment of FIGS. 8 and 9, therefore, at least the secondsubstrate assembly is located dry within the housing prior to applyingthe fixing material that fixes the second substrate assembly in place.The first substrate assembly and the spacer similarly may be applieddry. After locating the first and second substrate assemblies and spacerwithin the housing, portions of the housing are located spaced apartfrom an edge of the second substrate assembly to form fixing ports forapplying the fixing material, and the fixing material is applied throughthe fixing ports. The configuration of the fixing ports 128 (includingports 128 a and 128 b) may be varied, so long as the ports allow forglue or other adhesive to be delivered to the correct part of the EWODdevice during assembly. For example, the fixing ports 128 may be formedas openings adjacent to the bottom substrate assembly 84, as shown inFIG. 10. It also may be advantageous to apply the glue or other adhesiveremotely from the EWOD substrates into ports 128 and allow the glue torun along tracks 130 in the plastic housing that lead to the edge of thebottom substrate assembly, as shown in FIG. 11. The tracks optionallycan be inclined so that the glue or adhesive preferentially runs towardsthe EWOD substrates.

Relatedly, it may be advantageous to be use different types of adhesivesin different areas of the device. For example, it may be advantageous touse a flexible adhesive to adhere the upper substrate to the plastichousing, to accommodate any difference in thermal expansion between theglass and the plastic, whereas it may be preferable to use a differenttype of adhesive between the housing and the spacer and/or the spacerand the bottom substrate, as the fixing material in these areas may havean impact on the cell gap of the EWOD channel. In particular, it may beuseful to use an adhesive with low thermal expansion in these regions ofthe device.

FIG. 12 is a drawing depicting a side cross-section view of anotherexemplary EWOD device 80 e in accordance with embodiments of the presentinvention, which bears structural similarity to the embodiment of FIG.3. In this embodiment, however, a second fixing layer 132 is formeddifferently as compared to the second fixing layer 96 of FIG. 3. Rather,in this embodiment the second fixing layer 132 is formed to fix thelower substrate assembly 84 into position on its bottom outer surface134 to the housing 92, as illustrated in FIG. 12. This adhesion isbetter achieved using a fixing material selected from those describedabove that is a high viscosity glue or adhesive that can form a beadthat adheres the bottom surface 134 of the lower substrate 84 to thebottom outer surface 106 of the plastic housing 92, thereby holding thebottom substrate in position. This embodiment has an advantage in thatit is not necessary to pattern a hydrophobic layer on the upper surfaceof the lower substrate to hold the EWOD device together in a robustfashion. Rather, there simply should not be any hydrophobic coating onthe bottom outer surface of the lower substrate assembly at points atwhich the high viscosity glue or adhesive is applied.

Comparable principles may be applied to the EWOD device configuration ofFIG. 7, in which the housing and the bottom substrate are fixed toopposing faces of the spacer. Accordingly, FIG. 13 is a drawingdepicting a side cross-section view of another exemplary EWOD device 80f in accordance with embodiments of the present invention, which bearsstructural similarity to the embodiment of FIG. 7. Similarly as in FIG.12, in the embodiment of FIG. 13 the second fixing layer 132 is formedto fix the lower substrate assembly 84 into position at its bottomsurface 134 to the bottom outer surface 106 of the housing 92 using ahigh viscosity glue or adhesive of sufficient viscosity to form a bead.In this example, the spacer 114 is configured with a double-sided tapeconfiguration as referenced above, and so the specific fixing layers 116and 118 shown in FIG. 7 optionally are absent as shown in the exampleshown in FIG. 13.

The embodiments of FIGS. 12 and 13 have an advantage in that the glassparts of the device (upper and lower substrate assemblies) may be formedat a mother-glass level, and then the mother-glass may be diced up intoindividual glass modules before assembling the glass modules into theplastic housing. Fixing material ports (similar to ports 128 at shown inFIGS. 8-11) may be provided to direct the more viscous fixing materialused in the embodiments of FIGS. 12 and 13.

The design of the plastic housing could either be so that the bottomouter surface 106 of the plastic housing is substantially flush with thebottom surface 134 of the lower EWOD substrate 84, as in FIGS. 12 and13, or so that the bottom outer surface 106 of the plastic housingextends substantially beyond the bottom surface 134 of the lower EWODsubstrate 84. Such a configuration is shown for example in FIG. 14,which depicts an EWOD device 80 g comparable to EWOD device 80 e of FIG.12, and in FIG. 15, which depicts and EWOD device 80 h comparable toEWOD device 80 f of FIG. 13. With such configuration, fixing layer 132takes the form of a bead of fixing material that does not protrude belowthe bottom edge of the complete EWOD device 80 g/80 h.

FIG. 16 is a drawing depicting a side cross-section view of anotherexemplary EWOD device 80 i in accordance with embodiments of the presentinvention. In this embodiment, a tolerance on the plastic housing issufficiently tight that the dry spacer is no longer needed to define theproper cell gap during device use, and hence all is that is needed is toadhere the top and bottom substrate assemblies 82 and 84 to the plastichousing 92, as shown in FIG. 16. This embodiment has an advantage inthat there is one less piece (i.e., the spacer is eliminated) in thefinal EWOD device assembly, reducing cost and complexity.

As a corresponding method of assembly for the embodiment of FIG. 16, thedevice structure is initially assembled into the structure shown forexample in FIG. 3 with the spacer provided. In this manner, the spacercan set the substrate spacing to define the proper EWOD channel cell gapduring assembly, but the spacer does not remain inside the assembledEWOD device as part of the device during usage. Rather, the spacer actsas a tool to aid assembly, and the spacer subsequently is removed afterthe fixing layers have been cured, after which the resultant EWOD device80 i is configured as depicted in FIG. 16. In this embodiment, thefixing layer material is selected and applied in a manner that ensuresthat the fixing material does not wick underneath the spacer duringassembly, which could interfere with subsequent removal of the spacerafter curing. This embodiment has an advantage in that a uniform cellgap is produced similarly as when a dry spacer is utilized without theneed for retention of the spacer in the final device configuration.

FIG. 17 is a drawing depicting a side cross-section view of anotherexemplary EWOD device 80 j in accordance with embodiments of the presentinvention. This embodiment provides another manner in which deviceconfiguration may be simplified and less costly by the elimination of amore conventional component. In the embodiment of FIG. 17, a housing 140is formed having a portion 142 constituting an integral upper substrateassembly. In other words, in this embodiment the upper substrate 142 isnot separate from the remainder of the plastic housing 140. Accordingly,as integral components, the upper substrate 142 includes plastic and atleast in part is made of the same material as the overall housing 140.This embodiment has an advantage of reducing the number of parts andassembly steps in the production of the EWOD device, and thereforereduces the cost and complexity of the EWOD device.

The descriptions of the various configurations of EWOD device 80-80 jare non-specific as to any particular fluid input structures. FIG. 18 isa drawing depicting a plan view of the exemplary EWOD device 80 inaccordance with embodiments of the present invention, including theaddition of fluid input structures 144. In the example of FIG. 18, fluidinput structures may be formed as apertures in the upper substrate 82,which may be drilled or otherwise cut into such substrate. The use ofapertures 144 has an advantage of enabling the loading and extracting offluids from the EWOD device without using a specialized spacer design.It will be appreciated, however, that any suitable configuration ofinput structures may be employed, including for example configuring thespacer to permit side loading of fluid into the EWOD channel withouthaving to form or use apertures in the upper substrate.

An aspect of the invention, therefore, is an enhanced EWOD device havinga housing with alignment features optimized for simplified assembly. Inexemplary embodiments, the EWOD device includes: a first substrateassembly and a second substrate assembly; wherein one of the first orsecond substrate assemblies includes electrowetting electrodes, and thefirst substrate assembly and the second substrate assembly are spacedapart to define a channel between the first and second substrateassemblies; a housing for receiving the first substrate assembly and thesecond substrate assembly, the housing comprising an alignment featurefor locating at least one of the first and second substrate assemblieswithin the housing; and a fixing feature for fixing the first and secondsubstrate assemblies within the housing. The second substrate assemblyis located within the housing such that the second substrate assembly isan outer component of the EWOD device. The EWOD device may include oneor more of the following features, either individually or incombination.

In an exemplary embodiment of the EWOD device, the alignment featurecomprises a stepped configuration including a first substrate alignmentsurface for locating the first substrate assembly within the housing,and a second substrate alignment surface for locating the secondsubstrate assembly within the housing.

In an exemplary embodiment of the EWOD device, the fixing featurecomprises a first fixing layer of a fixing material that fixes the firstsubstrate assembly to the housing at the first substrate alignmentsurface.

In an exemplary embodiment of the EWOD device, the fixing featurecomprises a second fixing layer of a fixing material that fixes thesecond substrate assembly to the housing at the second substratealignment surface.

In an exemplary embodiment of the EWOD device, the fixing material is acurable adhesive or curable glue.

In an exemplary embodiment of the EWOD device, the fixing material has aviscosity sufficiently high to form beads of fixing material.

In an exemplary embodiment of the EWOD device, the EWOD device furtherincludes a spacer that spaces apart the first substrate assembly fromthe second substrate assembly to define the channel between the firstand second substrate assemblies; wherein the spacer is located betweenthe first and substrate assemblies out of direct contact with thehousing along at least a portion of the spacer.

In an exemplary embodiment of the EWOD device, the EWOD device furtherincludes a spacer that spaces apart the first substrate assembly fromthe second substrate assembly to define the channel between the firstand second substrate assemblies. The alignment feature comprises astepped configuration including a first substrate alignment surface forlocating the first substrate assembly within the housing, and a spaceralignment surface for locating the spacer within the housing; and thefixing feature fixes the second substrate assembly to the housingindirectly by fixing the second substrate assembly to the spacer.

In an exemplary embodiment of the EWOD device, the fixing featurecomprises a first fixing layer of fixing material that fixes the firstsubstrate assembly to the housing at the first substrate alignmentsurface.

In an exemplary embodiment of the EWOD device, the fixing featurefurther comprises a second fixing layer of fixing material that fixesthe spacer to the housing at the spacer alignment surface, and a thirdfixing layer of fixing material that fixes the second substrate assemblyto the spacer.

In an exemplary embodiment of the EWOD device, the spacer has adouble-sided tape configuration with adhesive layers being provided onopposing surfaces of a base spacer body to form the fixing feature.

In an exemplary embodiment of the EWOD device, portions of the housingare spaced apart from an edge of the second substrate assembly to formfixing ports for applying the fixing material.

In an exemplary embodiment of the EWOD device, a bottom outer surface ofthe housing extends beyond a bottom outer surface of the secondsubstrate assembly.

In an exemplary embodiment of the EWOD device, the first substrateassembly is integral as part of the housing.

Another aspect of the invention is a simplified method of assembling theEWOD device using a housing with alignment features optimized forsimplified assembly. In exemplary embodiments, the method of assemblingincludes the steps of: providing a housing for receiving a firstsubstrate assembly and a second substrate assembly, the housingcomprising an alignment feature for locating at least one of the firstand second substrate assemblies within the housing; locating the firstsubstrate assembly within the housing; locating the second substrateassembly within the housing, wherein at least one of the first andsecond substrate assemblies is located within the housing using thealignment feature; and applying a fixing material to form at least onefixing layer to fix the first and second substrate assemblies within thehousing. One of the first or second substrate assemblies includeselectrowetting electrodes, and the first substrate assembly and thesecond substrate assembly are spaced apart to define a channel betweenthe first and second substrate assemblies. The second substrate assemblyis located within the housing such that the second substrate assembly isan outer component of the EWOD device. The method of assembling mayinclude one or more of the following features, either individually or incombination.

In an exemplary embodiment of the method of assembling, the methodfurther includes locating a spacer within the housing, wherein thespacer spaces apart the first substrate assembly from the secondsubstrate assembly to define the channel between the first and secondsubstrate assemblies.

In an exemplary embodiment of the method of assembling, the secondsubstrate assembly is located within the housing prior to applying thefixing material that fixes the second substrate assembly in place; afterlocating the first and second substrate assemblies within the housing,portions of the housing are located spaced apart from an edge of thesecond substrate assembly to form fixing ports for applying the fixingmaterial; and the fixing material is applied through the fixing ports.

In an exemplary embodiment of the method of assembling, the methodfurther includes locating a spacer within the housing; wherein thespacer spaces apart the first substrate assembly from the secondsubstrate assembly to define the channel between the first and secondsubstrate assemblies; and the spacer is located within the housing priorto applying the fixing material that fixes the spacer and/or secondsubstrate assembly in place.

In an exemplary embodiment of the method of assembling, the methodincludes: providing the housing; applying fixing material to form afirst fixing layer at a first alignment surface defined by the housing;locating the first substrate assembly within the housing using the firstalignment surface to fix the first substrate assembly to the housing viathe first fixing layer; locating a spacer adjacent to the firstsubstrate assembly; applying fixing material to form a second fixinglayer at a second alignment surface defined by the housing; and locatingthe second substrate assembly within the housing using the secondalignment surface to fix the second substrate assembly to either thehousing or the spacer via the second fixing layer, wherein the spacerspaces apart the first substrate assembly from the second substrateassembly to define the channel between the first and second substrateassemblies.

In an exemplary embodiment of the method of assembling, the fixingmaterial is a curable adhesive or curable glue, the method furtherincluding curing the EWOD device to set the fixing material.

Although the invention has been shown and described with respect to acertain embodiment or embodiments, equivalent alterations andmodifications may occur to others skilled in the art upon the readingand understanding of this specification and the annexed drawings. Inparticular regard to the various functions performed by the abovedescribed elements (components, assemblies, devices, compositions,etc.), the terms (including a reference to a “means”) used to describesuch elements are intended to correspond, unless otherwise indicated, toany element which performs the specified function of the describedelement (i.e., that is functionally equivalent), even though notstructurally equivalent to the disclosed structure which performs thefunction in the herein exemplary embodiment or embodiments of theinvention. In addition, while a particular feature of the invention mayhave been described above with respect to only one or more of severalembodiments, such feature may be combined with one or more otherfeatures of the other embodiments, as may be desired and advantageousfor any given or particular application.

INDUSTRIAL APPLICABILITY

The described embodiments could be used to provide an enhanced AM-EWODdevice. The AM-EWOD device could form a part of a lab-on-a-chip system.Such devices could be used in manipulating, reacting and sensingchemical, biochemical or physiological materials. Applications includehealthcare diagnostic testing, material testing, chemical or biochemicalmaterial synthesis, proteomics, tools for research in life sciences andforensic science.

REFERENCE SIGNS LIST

-   10—lower substrate-   12—array element electrodes-   12A—individual array element electrode-   12B—individual array element electrode-   14—liquid droplet-   16—top substrate-   18—spacer-   20—nonpolar surround fluid-   22—insulator layer-   24—first hydrophobic coating-   26—contact angle-   28—second hydrophobic coating-   30—reference electrode-   36—exemplary AM-EWOD device-   44—lower substrate-   46—thin film electronics-   48—array element electrodes-   50—element array-   52—liquid droplet-   54—upper substrate-   56—spacer-   80/80 a-j—EWOD device-   82—upper substrate assembly-   84—lower substrate assembly-   86—spacer-   88—EWOD channel-   90—regions of spacer-   92—housing-   94—first fixing layer-   96—second fixing layer-   100—first substrate alignment surface-   102—second substrate alignment surface-   106—bottom outer surface-   110—housing-   112—first fixing layer-   114—spacer-   116—second fixing layer-   118—third fixing layer-   120—first substrate alignment surface-   122—spacer alignment surface-   124—portions of housing-   125—fixing material-   126—edge of bottom substrate-   128—fixing ports-   128 a—first fixing ports-   128 b—second fixing ports-   132—second fixing layer-   134—bottom outer surface-   140—housing-   142—upper substrate-   144—fluid input structures

1. An electrowetting on dielectric (EWOD) device comprising: a firstsubstrate assembly and a second substrate assembly; wherein one of thefirst or second substrate assemblies includes electrowetting electrodes,and the first substrate assembly and the second substrate assembly arespaced apart to define a channel between the first and second substrateassemblies; a housing for receiving the first substrate assembly and thesecond substrate assembly, the housing comprising an alignment featurefor locating at least one of the first and second substrate assemblieswithin the housing; and a fixing feature for fixing the first and secondsubstrate assemblies within the housing. wherein the second substrateassembly is located within the housing such that the second substrateassembly is an outer component of the EWOD device.
 2. The EWOD device ofclaim 1, wherein the alignment feature comprises a stepped configurationincluding a first substrate alignment surface for locating the firstsubstrate assembly within the housing, and a second substrate alignmentsurface for locating the second substrate assembly within the housing.3. The EWOD device of claim 2, wherein the fixing feature comprises afirst fixing layer of a fixing material that fixes the first substrateassembly to the housing at the first substrate alignment surface.
 4. TheEWOD device of claim 2, wherein the fixing feature comprises a secondfixing layer of a fixing material that fixes the second substrateassembly to the housing at the second substrate alignment surface. 5.The EWOD device of claim 3, wherein the fixing material is a curableadhesive or curable glue.
 6. The EWOD device of claim 5, wherein thefixing material has a viscosity sufficiently high to form beads offixing material.
 7. The EWOD device of claim 1, further comprising aspacer that spaces apart the first substrate assembly from the secondsubstrate assembly to define the channel between the first and secondsubstrate assemblies; wherein the spacer is located between the firstand substrate assemblies out of direct contact with the housing along atleast a portion of the spacer.
 8. The EWOD device of claim 1, furthercomprising a spacer that spaces apart the first substrate assembly fromthe second substrate assembly to define the channel between the firstand second substrate assemblies; wherein: the alignment featurecomprises a stepped configuration including a first substrate alignmentsurface for locating the first substrate assembly within the housing,and a spacer alignment surface for locating the spacer within thehousing; and the fixing feature fixes the second substrate assembly tothe housing indirectly by fixing the second substrate assembly to thespacer.
 9. The EWOD device of claim 8, wherein the fixing featurecomprises a first fixing layer of fixing material that fixes the firstsubstrate assembly to the housing at the first substrate alignmentsurface.
 10. The EWOD device of claim 8, wherein the fixing featurefurther comprises a second fixing layer of fixing material that fixesthe spacer to the housing at the spacer alignment surface, and a thirdfixing layer of fixing material that fixes the second substrate assemblyto the spacer.
 11. The EWOD device of claim 8, wherein the spacer has adouble-sided tape configuration with adhesive layers being provided onopposing surfaces of a base spacer body to form the fixing feature. 12.The EWOD device of claim 9, wherein portions of the housing are spacedapart from an edge of the second substrate assembly to form fixing portsfor applying the fixing material.
 13. The EWOD device of claim 1,wherein a bottom outer surface of the housing extends beyond a bottomouter surface of the second substrate assembly.
 14. The EWOD device ofclaim 1, wherein the first substrate assembly is integral as part of thehousing.
 15. A method of assembling an electrowetting on dielectric(EWOD) device comprising the steps of: providing a housing for receivinga first substrate assembly and a second substrate assembly, the housingcomprising an alignment feature for locating at least one of the firstand second substrate assemblies within the housing; locating the firstsubstrate assembly within the housing; locating the second substrateassembly within the housing, wherein at least one of the first andsecond substrate assemblies is located within the housing using thealignment feature; and applying a fixing material to form at least onefixing layer to fix the first and second substrate assemblies within thehousing; wherein one of the first or second substrate assembliesincludes electrowetting electrodes, and the first substrate assembly andthe second substrate assembly are spaced apart to define a channelbetween the first and second substrate assemblies; and wherein thesecond substrate assembly is located within the housing such that thesecond substrate assembly is an outer component of the EWOD device. 16.The method of assembling of claim 15, further comprising locating aspacer within the housing, wherein the spacer spaces apart the firstsubstrate assembly from the second substrate assembly to define thechannel between the first and second substrate assemblies.
 17. Themethod of assembling of claim 15, wherein: the second substrate assemblyis located within the housing prior to applying the fixing material thatfixes the second substrate assembly in place; after locating the firstand second substrate assemblies within the housing, portions of thehousing are located spaced apart from an edge of the second substrateassembly to form fixing ports for applying the fixing material; and thefixing material is applied through the fixing ports.
 18. The method ofassembling of claim 17, further comprising locating a spacer within thehousing; wherein the spacer spaces apart the first substrate assemblyfrom the second substrate assembly to define the channel between thefirst and second substrate assemblies; and the spacer is located withinthe housing prior to applying the fixing material that fixes the spacerand/or second substrate assembly in place.
 19. The method of assemblingof claim 15, comprising: providing the housing; applying fixing materialto form a first fixing layer at a first alignment surface defined by thehousing; locating the first substrate assembly within the housing usingthe first alignment surface to fix the first substrate assembly to thehousing via the first fixing layer; locating a spacer adjacent to thefirst substrate assembly; applying fixing material to form a secondfixing layer at a second alignment surface defined by the housing; andlocating the second substrate assembly within the housing using thesecond alignment surface to fix the second substrate assembly to eitherthe housing or the spacer via the second fixing layer, wherein thespacer spaces apart the first substrate assembly from the secondsubstrate assembly to define the channel between the first and secondsubstrate assemblies.
 20. The assembling method of claim 15, wherein thefixing material is a curable adhesive or curable glue, the methodfurther comprising curing the EWOD device to set the fixing material.